The present invention pertains to apparatus and method for enlarging subterranean holes using a substantially conical bit mounted at the end of a string of segmented drill pipe or flexible tubing.
In recent years the placement of utility service lines in shallow boreholes drilled substantially parallel to the earth""s surface has become more common. Initially, these lines were buried by digging a shallow trench, placing the utility service line in the trench, and then re-filling the trench. Later, it was realized that the horizontal and directional drilling techniques used in deep well drilling could be adapted for the drilling of shallow boreholes for utility lines. Now, it is common to see shallow borehole drilling machines, which include a rack of segmented pipe sections, drilling shallow boreholes for utility lines.
The drilling of shallow boreholes for utility lines is typically a two step process. First, a pilot hole is drilled outwardly from a location proximal to the drilling machine. When the shallow borehole reaches its predetermined destination or distal location, the drilling tool is steered upwardly towards the earth""s surface or caused to enter an exit pit. It is at this time that the drilling tool, which was used to drill the initial pilot hole is removed and replaced with a backreaming tool. The backreaming tool is then pulled back through the hole to enlarge the borehole to a desired size.
Typical to the subterranean hole boring industry are enlarged drill bits that are utilized in a back drilling, or xe2x80x9cbackreamingxe2x80x9d operation. These enlarged drill bits are known as backreamers. These backreamers are made in various sizes and shapes but are all based on a configuration wherein the drill face is pointed back toward the drilling machine located at the proximal point of the borehole. This configuration enables the bit face to cut while the drill pipe or tubing is pulled back towards the drilling equipment from the distal end of the borehole.
Prior art backreaming tools have been clumsy to use and inefficient. Accordingly, a need remains in the art for a backreaming tool which is easy to use and is efficient.
The apparatus and method of the present invention includes a backreaming tool assembly for use in enlarging shallow boreholes drilled in an earthen environment which is easy to use and efficient.
The backreamer assembly 10 of the present invention differs from conventional backreamers by the placement of a drive mechanism or transmission inside the backreamer cutting tool. A downhole motor on the end of the segmented drill pipe flexible tubing applies the required rotational force to rotate the cutting tool. The drive mechanism within the backreamer cutting tool is a transmission or gearbox. This allows the input power to the backreamer cutting tool to be increased, thereby increasing the output torque at the face of the backreamer cutting tool which encounters the earthen matter surrounding the hole as the backreamer cutting tool moves through and enlarges the borehole. These transmission gearboxes are of a standard helical or planetary design that provide a reduction in input speed and an increase in output torque.